Modern communications networks are increasingly based on silica optical fiber which offers very wide bandwidth within several spectral wavelength bands. At the transmitter end of a typical point-to-point fiber optic communications link an electrical data signal is used to modulate the output of a semiconductor laser emitting, for example, in the 1525-1565 nanometer transmission band (the so-called C-band), and the resulting modulated optical signal is coupled into one end of the silica optical fiber. On sufficiently long links the optical signal may be directly amplified along the route by one or more amplifiers, for example, optically-pumped erbium-doped fiber amplifiers (EDFAs). At the receiving end of the fiber link, a photodetector receives the modulated light and converts it back to its original electrical form. For very long links the optical signal risks becoming excessively distorted due to fiber-related impairments such as chromatic and polarization dispersion and by noise limitations of the amplifiers, and may be reconstituted by detecting and re-launching the signal back into the fiber. This process is typically referred to as optical-electrical-optical (OEO) regeneration.
In recent developments, the transmission capacity of fiber optic systems has been greatly increased by wavelength division multiplexing (WDM) in which multiple independent optical signals, differing uniquely by wavelength, are simultaneously transmitted over the fiber optic link. For example, the C-band transmission window has a bandwidth of about 35 nanometers, determined partly by the spectral amplification bandwidth of an EDFA amplifier, in which multiple wavelengths may be simultaneously transmitted. All else being equal, for a WDM network containing a number N wavelengths, the data transmission capacity of the link is increased by a factor of N. Depending on the specifics of a WDM network, the wavelength multiplexing into a common fiber is typically accomplished with devices employing a dispersive element, an arrayed waveguide grating, or a series of thin-film filters. At the receiver of a WDM system, the multiple wavelengths can be spatially separated using the same types of devices that performed the multiplexing and then separately detected and output in their original electrical data streams.
Dense WDM (DWDM) systems are being designed in which the transmission spectrum includes 40, 80, or more wavelengths with wavelength spacing of less than 1 nanometer. Current designs have wavelength spacing of between 0.4 and 0.8 nanometer, or equivalently a frequency spacing of 50 to 100 GHz respectively. Spectral packing schemes allow for higher or lower spacing, dictated by economics, bandwidth, and other factors. Other amplifier types, for example Raman, that help to expand the available WDM spectrum are currently being commercialized. However, the same issues about signal degradation and OEO regeneration exist for WDM as with non-WDM fiber links. The expense of OEO regeneration is compounded by the large number of wavelengths present in WDM systems.
Modern fiber optic networks are evolving to be much more complicated than the simple point-to-point “long haul” systems described above. Instead, as fiber optic networks move into the regional, metro, and local arenas, they increasingly include multiple nodes along the fiber span, and connections between fiber spans (e.g., mesh networks and interconnected ring networks) at which signals received on one incoming link can be selectively switched between a variety of outgoing links, or taken off the network completely for local consumption. For electronic links, or optical signals that have been detected and converted to their original electrical form, conventional electronic switches directly route the signals to their intended destination, which may then include converting the signals to the optical domain for fiber optic transmission. However, the desire to switch fiber optic signals while still in their optical format, thereby avoiding expensive OEO regeneration to the largest extent possible, presents a new challenge to the switching problem. Purely optical switching is generally referred to as all-optical or OOO switching optical/optical (OOO).
Switching
In the most straightforward and traditional fiber switching approach, each network node that interconnects multiple fiber links includes a multitude of optical receivers which convert the signals from optical to electrical form, a conventional electronic switch which switches the electrical data signals, and an optical transmitter which converts the switched signals from electrical back to optical form. In a WDM system, this optical/electrical/optical (OEO) conversion must be performed by separate receivers and transmitters for each of the W wavelength components on each fiber. This replication of expensive OEO components is currently slowing the implementation of highly interconnected mesh WDM systems employing a large number of wavelengths.
Another approach for fiber optic switching implements sophisticated wavelength switching in an all-optical network. In a version of this approach that may be used with the present invention, the wavelength components W from an incoming multi-wavelength fiber are demultiplexed into different spatial paths. Individual and dedicated switching elements then route the wavelength-separated signals toward the desired output fiber port before a multiplexer aggregates the optical signals of differing wavelengths onto a single outgoing fiber. In conventional fiber switching systems, all the fiber optic switching elements and associated multiplexers and demultiplexers are incorporated into a wavelength selective switch (WSS), which is a special case of an enhanced optical cross connect (OXC) having a dispersive element and wavelength-selective capability. Additionally, such systems may incorporate lenses and mirrors which focus light and lenslets which collimate such light.
Advantageously, all the fiber optic switching elements can be implemented in a single chip of a micro electromechanical system (MEMS). The MEMS chip generally includes a two-dimensional array of tiltable mirrors which may be separately controlled. U.S. Pat. No. 6,097,859 to Solgaard et al., incorporated herein in its entirety, describes the functional configuration of such a MEMS wavelength selective switch (WSS), which incorporates a wavelength from an incoming fiber and is capable of switching wavelength(s) to any one of multiple outgoing fibers. The entire switching array of several hundred micro electromechanical system (MEMS) mirrors can be fabricated on a chip having dimension of less than one centimeter by techniques well developed in the semiconductor integrated circuit industry.
Solgaard et al. further describes a large multi-port (including multiple input M and multiple output N ports) and multi-wavelength WDM wavelength selective switch (WSS), accomplishing this by splitting the WDM channels into their wavelength components W and switching those wavelength components W. The Solgaard et al. WSS has the capability of switching any wavelength channel on any input port to the corresponding wavelength channel on any output fiber port. Again, a wavelength channel on any of the input fibers can be switched to the same wavelength channel on any of the output fibers. Each MEMS mirror in today's WDM wavelength selective switch is dedicated to a single wavelength channel whether it tilts about one or more axis.
A complex WDM or white-light network is subject to many problems. For example, the different optical signals which are propagating on a particular link or being optically processed may have originated from different sources across the network. Also, in a WDM system, the WDM wavelength output power may vary from transmitter to transmitter because of environmental changes, aging, or differences in power injected into the WDM stream. Different optical sources for either a WDM or white-light system are additionally subject to different amounts of attenuation over the extended network. Particularly, for a wavelength-routed transparent network, the WDM spectrum on a given fiber contains wavelength components which generally have traversed many diverse paths from different sources and with different losses and different impairment accumulation, such as degradation of the optical signal-to-noise ratio or dispersion broadening. Further, wavelength multiplexing and demultiplexing usually rely on optical effects, such as diffraction or waveguide interference, which are very sensitive to absolute wavelength, and which cannot be precisely controlled. Additionally, the prior art is disadvantageously limited to complex multi input and output fiber port, single dedicated wavelength channel MEMS mirrors, and multi wavelength WDM cross-connect switches.
EDFAs or other optical amplifiers may be used to amplify optical signals to compensate loss, but they amplify the entire WDM signal and their gain spectrum is typically not flat. Therefore, measures are needed to maintain the power levels of different signals at common levels or at least in predetermined ratios.
Monitoring
Monitoring of the WDM channels is especially important in optical telecommunication networks that include erbium doped fiber amplifiers (EDFAs), because a power amplitude change in one channel may degrade the performance of other channels in the network due to gain saturation effects in the EDFA. Network standard documents, such as the Bellcore GR-2918, have been published to specify wavelength locations, spacing and signal quality for WDM channels within the networks. Network performance relative to these standards can be verified by monitoring wavelength, power and signal-to-noise ratio (SNR) of the WDM channels.
A multi-wavelength detector array or spectrometer may be integrated into the free space of a WSS and utilized to monitor wavelength channels, power and signal-to-noise ratio (SNR) in telecommunication networks. Typically, a portion of the WDM channels are diverted by a splitter or partially reflective mirror to an optical power monitor or spectrometer to enable monitoring of the WDM channels. Each MEMS mirror in today's WDM wavelength selective switch (WSS) is dedicated to a single wavelength channel. Whether it tilts about one or more axes, such mirror may be used to control the amount of optical power passing through WSS for such single wavelength channel. In addition, a detector array or spectrometer may be external to the free space of the WSS or OXC, and may be utilized to monitor white light (combined wavelength channels) power, and signal-to-noise ratio of optical signal via input/output fiber port taps or splitters. More specifically, the prior art consists of costly large two-dimensional detector arrays or spectrometer utilized to monitor multiple input or output wavelength channels, power and signal-to-noise ratio.
Monitoring and switching are part of a feedback loop required to achieve per-wavelength insertion loss control and such systems comprise three classic elements: sensor for monitoring, actuator for multi wavelength switching and attenuating, and processor for controlling wavelength switching, selection and equalization. The actuator in today's WSS products is typically a MEMS-based micromirror or a liquid crystal blocker or reflector. The sensor is typically a modular optical power monitor, comprising a mechanical filter for wavelength selection and a photodetector. Depending on the system, the three elements can be co-located in the same device, or can exist as separate standalone cards connected by a backplane.
In general, higher levels of integration of the sensor, actuator, and processor are attractive from a size, cost, speed, and simplicity of operation standpoint. The proposed new solution reaps these benefits because of a very high level of integration.
Therefore, it is readily apparent that there would be a recognizable benefit from a cost effective fiber optic switch with dual channel selector for all-optical communication networks in which each switching node demultiplexes the aggregate multi-wavelength WDM signal from input fibers into its wavelength components, spatially switches one of many single-wavelength components from different input fibers for each wavelength channel, and wherein such switch multiplexes the switched wavelength components to one output fiber for retransmission; and wherein such wavelength components power may be monitored and varied by controllable attenuation, resulting in a higher level of integration of the sensor, actuator, and processor and enabling multiple switches in a single device capable of utilizing common optical components.